Two astronauts will make a hastily planned spacewalk on Saturday to try to fix an ammonia leak in the International Space Station's power system.

The leak in a cooling system was discovered on Thursday when "snowflakes" of ammonia were seen flying away from the station.

Chris Hadfield, onboard the ISS has tweeted:

&lt;noframe&gt;Twitter: Chris Hadfield - Good Morning, Earth! A complex &amp;amp; vital day on Space Station, as Tom and Chris suit up and go outside to help fix the ammonia leak. Busy day!&lt;/noframe&gt;

Spacewalks are rarely done on such short notice, but the US space agency wanted to address the leak before all the ammonia escaped. They also want to take advantage of a spacewalking crew member who is about to return home.

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Officials said the space station has plenty of power, even though the leak forced Nasa to shut off the power channel from one of eight solar panels that supply electricity.

The station can operate fine with only seven electrical channels, space station programme manager Michael Suffredini said on Friday. Power from the affected panel was re-routed to the other seven systems.

Suffredini said the chief suspect for the leak is space junk hitting a cooling tube, but he said the area had a slow small leak for many years that suddenly accelerated on Thursday.

"You're talking a very, very, very small hole," Suffredini said at a news conference.

While he described it as a "serious situation," he characterised it more as an annoyance.

Nasa hopes the leak is in a small pump box. During the six-hour spacewalk on Saturday, US astronauts Chris Cassidy and Tom Marshburn will replace the 260-pound box with a nearby spare.

While Nasa has had to do impromptu spacewalks before, they have not been done on the space station since it was completely built and operating as a finished lab, said chief flight director Norm Knight.

Station Commander Chris Hadfield of Canada told Nasa flight controllers on Friday that the crew is completely ready for the spacewalk.

"It's the right thing to do," he radioed down to Earth.

Hadfield tweeted that the crew was working "like clockwork" and said the two spacewalkers were already getting their spacesuits ready.

If the cooling system cannot be fixed in Saturday's spacewalk, it can be fixed in later spacewalks, Suffredini said. Nasa can and has operated the station fully on seven power channels, he said.

But that would leave the station little margin for error. If there are more problems, some experiments on board may have to be shut down to conserve power.

Nasa spokesman Rob Navias said the repair is what the agency calls one of the "Big 12" types of emergency repair work that all spacewalking astronauts train for in advance.

In 2009, Cassidy and Marshburn flew to the space station on the shuttle Endeavour and walked in space together to swap out a battery in the same location, so "they know this worksite inside and out," Navias said.

Marshburn, Hadfield and Russia's Roman Romanenko are set to return to Earth on Monday. Nasa Administrator Charles Bolden told reporters on Friday that their return will go ahead as planned.